The present invention relates to a method for fabricating metal interconnections used for flat panel displays such as liquid crystal displays (LCD), plasma display panels (PDP), electrochromic displays (ECD) and electroluminescent displays (ELD), as well as for printed wiring boards using ceramic substrates, or in other various fields.
In a flat panel display typified by LCDs, normally, display material such as liquid crystals or discharge gas is sandwiched between a pair of substrates, and a voltage is applied to the display material. In this case, metal interconnection lines made of electrically conductive material are arrayed on at least one of the substrates.
For example, in the case of an active matrix drive type LCD, gate electrodes and data electrodes are disposed in a matrix shape on one substrate (active matrix substrate), which is one of the pair of substrates between which the display material is sandwiched and held. A thin film transistor (TFT) and a pixel electrode are provided at each intersection of those electrodes. Generally, these gate electrodes and data electrodes are made of metal material such as Ta, Al or Mo, and deposited by a dry deposition technique such as vapor deposition, sputtering, or Chemical Vapor Deposition (CVD) process.
When making an attempt to obtain such flat panel displays having a larger area and/or a fine structure, the drive frequency is increased and thereby the resistance of metal interconnections and the parasitic capacitance increase. As a result of this, delay of driving signals comes up as a significant drawback.
Thus, in order to dissolve the delay of driving signals, there has been made an attempt to use Cu having lower electrical resistance (bulk resistivity: 1.7 .mu..OMEGA..multidot.cm) as the interconnection material, instead of Al (bulk resistivity: 2.7 .mu..OMEGA..multidot.cm), .alpha.-Ta (bulk resistivity: 13.1 .mu..OMEGA..multidot.cm) or Mo (bulk resistivity: 5.8 .mu..OMEGA..multidot.cm), which are conventional interconnection materials. As a method for fabricating such metal interconnections, for example, the following (1) and (2) are available:
(1) "Low Resistance Copper Address Line for TFT-LCD" (Japan Display '89, pp. 498-501) discloses discussion results on a TFT-LCD in which Cu is used as the gate electrode material. According to this literature, it is expressly described that because a Cu film (low-resistance metal film) deposited by sputtering process with the aim of lowering the resistance has poor adhesion to the ground glass substrate, a ground metal film such as a metal film of Ta or the like deposited by sputtering process needs to be interveniently provided between the Cu film and the ground glass substrate in order to enhance the adhesion. PA1 (2) Japanese Patent Laid-Open Publication HEI 2-83533 discloses a method of forming the metal interconnections of Cu by using a plating deposition technique without using any dry deposition technique such as sputtering process. In this case, to solve the poor adhesion of Cu film (low-resistance metal film) to the ground oxide (ITO), this method adopts metal interconnections of a Cu/Au/Ni layered structure in which a Ni film (ground metal film) and a Au film (anti-corrosion metal film) deposited. by electroless plating are interveniently provided between the Cu film and the ground oxide.
However, the metal interconnections shown above have the following drawbacks.
In the prior art example (1), in the process of forming a Cu film and a Ta film or the like by a dry deposition technique such as sputtering process for the formation of a Cu/Ta layered film, dry deposition process and etching process are necessary for the Cu film and the Ta film or the like, separately. This causes the number of processes to increase, which leads to a disadvantage of cost increase.
In the prior art example (2), the process of forming a Cu/Au/Ni layered film by plating deposition techniques needs to use electroless plating process for the Ni film. This is because when a metal is plated on an insulating substrate of glass or on an oxide film, a metal film is deposited generally by using electroless plating after a catalyst such as Pd is stuck on the insulating substrate or the oxide film. However, in the presence of a catalyst concentrated portion, i.e., in a poor dispersion of catalyst, there would occur abnormal growth of the Ni film at places where the poor dispersion of catalyst is present. This would inadversely cause minute protrusions on the surface of the deposited Ni film.
FIG. 4 is a schematic sectional view of a Ni film 102 formed by electroless plating on a surface of a glass substrate 101 on which Pd catalyst has been provided. FIG. 4 shows that part of the columnar-grown Ni film 102, where the Pd catalyst is concentrated, is abnormally grown to form a protrusion 103. Such a protrusion failure shown in FIG. 4 is often seen in the process of electroless plating, which is attributed to an effect of the particle size or dispersibility of the Pd catalyst.
Also, in normal plating techniques, even depending on differences in composition, pH, temperature, etc. of the plating bath, the resulting Ni film varies in crystallinity or deposition state. In some cases, the Ni film results in quite a meager state or a sparse state. In such a case, pinholes are prone to occur in the Ni film. When such a poor-quality Ni film is used as the ground, the Cu/Au film stacked on the Ni film is more prone to occurrence of local film floats, so-called "swelling" failures, corresponding to the pinholes of the Ni film.
On this account, in order to avoid adverse effects of film quality failures of pinholes or the like of the Ni film on the upper-layer Cu/Au film, the prior art example (2) describes a method of forming the Ni film of 0.4 .mu.m or more in thickness, the Au film of 0.1 .mu.m or more, and the Cu film of 0.8 .mu.m or more. As a result of this, the total film thickness of metal interconnections composed of the Cu/Au/Ni layered film is 1 .mu.m or more inevitably. However, because the prior art example (2) was based on the presumption that the metal interconnections are used in peripheral terminal part of a liquid crystal panel, the increase in the film thickness of the metal interconnections was not considered as an issue.
When the metal interconnections are applied not only to peripheral terminal part of the liquid crystal panel but also to bus lines (scan lines and signal lines) within the display area or the like, the increase in film thickness of metal interconnections causes the following drawbacks:
Firstly, in the case of a device structure in which other metal interconnections or thin films are formed on the above metal interconnections, there is a drawback that the other metal interconnections or the thin films cannot fully cover a step gap corresponding to the film thickness of the metal interconnections, i.e., a step gap between the insulating substrate and the metal interconnections, so that disconnections of the other metal interconnections or breaks of the thin films are more prone to occur.
Secondly, when the metal interconnections are used as bus lines within the display area of the liquid crystal panel, there is a drawback that the step gap corresponding to the film thickness of the metal interconnections, i.e., the step gap between the insulating substrate and the metal interconnections is so large that there is a higher probability of occurrence of orientation disturbance of liquid crystal molecules.
Accordingly, in order to use the metal interconnections in a wider variety of applications, the total film thickness of metal interconnections formed of the Cu/Au/Ni layered film is preferably as thin as possible, and more specifically, desired to be designed as not more than 0.5 .mu.m. For implementation of such metal interconnections, it is indispensable to reduce the thickness of the ground Ni film, and improvement in the film quality of the Ni film is strongly demanded.
Therefore, an object of the present invention is to provide a method for fabricating metal interconnections, as well as a wiring board having the metal interconnections, capable of low cost fabrication by omitting dry deposition process or etching process, capable of preventing occurrence of minute protrusions on the surface of the ground metal film as compared with the prior art example (2), and moreover capable of thinning the film thickness.
In order to achieve the above object, present invention provides a method for fabricating metal interconnections, comprising: a first step of forming a first metal film on an insulating substrate by dry deposition technique; a second step of forming a second metal film selectively on the first metal film by wet deposition technique; and a third step of forming a third metal film selectively on the second metal film by wet deposition technique.
According to the invention, the dry deposition process has only. to be done one time of the first step. Also, the second metal film is formed selectively on the first metal film, and further the third metal film is formed selectively on the second metal film. Therefore, the patterning process (etching process) has only to be done one time for the first metal film, allowing the number of processes to be decreased, compared with the metal interconnection fabrication method shown in the prior art example (1). Thus, a cost reduction can be achieved.
Also, the first metal film is formed on the insulating substrate by using the dry deposition technique instead of the plating deposition technique involving catalyst impartment. Therefore, compared with the metal interconnection fabrication method shown in the prior art example (2), the pre-plating process for performing catalyst impartment is no longer necessary, so that occurrence of minute protrusions on the surface of the first metal film due to the catalyst can be avoided.
Further, since the first metal film is formed by the dry deposition technique, occurrence of pinholes is completely eliminated even if the first metal film is made thinner. As a result, the first metal film can be made thinner so that the total film thickness of metal interconnections can be made thinner.
In one embodiment of the invention, the wet deposition technique in the second step is electroplating technique or electroless plating technique.
According to the embodiment, the second metal film is formed by electroplating technique or electroless plating technique in the second step. Therefore, the second metal film can be formed only on the first metal film without performing the patterning process for the second metal film.
Also, when the wet deposition technique in the second step is electroless plating technique, the second metal film can be formed to a uniform thickness even if the area of the insulating substrate is increased.
In an embodiment of the invention, the wet deposition technique in the second step is displacement plating technique.
According to the embodiment, since the wet deposition technique in the second step is displacement plating technique, the pre-plating process for performing catalyst impartment is no longer necessary, so that the working efficiency in forming the second metal film can be improved.
In an embodiment of the invention, the wet deposition technique in the third step is electroplating technique or electroless plating technique.
According to the embodiment, the third metal film is formed by electroplating technique or electroless plating technique in the third step. Therefore, the third metal film can be formed only on the second metal film without performing the patterning process for the third metal film.
Also, when the wet deposition technique in the third step is electroless plating technique, the third metal film can be formed to a uniform thickness even if the area of the insulating substrate is increased.
In an embodiment of the invention, the wet deposition technique in the third step is electroless plating technique; and the second metal film has a catalytic action for deposition reaction of the third metal film.
According to the embodiment, the third metal film is formed by electroless plating in the third step. In this process, since the second metal film has catalytic action for the deposition reaction of the third metal film, the catalyst impartment of the pre-processing performed by electroless plating is no longer necessary so that the working efficiency in forming the third metal film can be improved.
In an embodiment of the invention, the first metal film is a metal film whose principal component is at least one of Ni, Ta, Mo, Cr, Ti and Al.
According to the embodiment, the first metal film contains, as a principal component, at least one of Ni, Ta, Mo, Cr, Ti and Al. Therefore, the first metal film can be formed on the insulating substrate easily with good adhesion by using a dry deposition technique such as sputtering process or vapor deposition process.
In an embodiment of the invention, the second metal film is a metal film whose principal component is a noble metal.
According to the embodiment, since the second metal film is a metal film whose principal component is a noble metal superior in corrosion resistance, oxide is less likely to be formed on the surface of the second metal film, which is the noble metal film. Therefore, no oxide is present between the second metal film and the third metal film that is formed on the second metal film, so that the third metal film can be formed on the second metal film with good adhesion by electroplating technique or electroless plating technique.
In an embodiment of the invention, the second metal film is a metal film whose principal component is Au.
According to the embodiment, since the second metal film contains, as a principal component, Au of low resistivity, the resistance value of the layered film composed of the first metal film and the second metal film becomes low, making it easier to pass electric current through the layered film. Accordingly, with the use of electroplating technique in the third step, since electric current is passed through the layered film composed of the first metal film and the second metal film, it is. advantageous that the layered film is of low resistance. Further, since Au is an expensive noble metal, making the second metal film as thin as possible and using an inexpensive material for the third metal film makes it possible to manufacture the metal interconnections with lower cost.
In an embodiment of the invention, the third metal film is a metal film whose principal component is at least one of Cu and Ag.
According to the embodiment, since the third metal film contains, as a principal component, at least one of Cu with a bulk resistivity of 1.7 .mu..OMEGA..multidot.cm and Ag with a bulk resistivity of 1.6 .mu..OMEGA..multidot.cm, the third metal film becomes lower in resistivity. Therefore, Cu and Ag are best suited as the material of low-resistance metal interconnections.
In particular, Cu is long in life against electromigration, and moreover inexpensive, thus superior in that the cost for manufacturing metal interconnections can be reduced.
In an embodiment of the invention, the method for fabricating metal interconnections further comprising: a fourth step of forming a cap film on the third metal film by wet deposition technique.
According to the embodiment, since the cap film is formed on the third metal film by wet deposition technique so as to cover the third metal film, the third metal film is prevented from being exposed to the atmospheric air. Therefore, the third metal film can be prevented from oxidizing.
In an embodiment of the invention, the wet deposition technique in the fourth step is electroplating technique or electroless plating technique.
According to the embodiment, since the cap film is formed by electroplating technique or electroless plating technique in the fourth step, the cap film can be formed only on the third metal film without performing the patterning process for the cap film.
Also, when the wet deposition technique in the fourth step is electroless plating technique, the cap film can be formed to a uniform thickness even if the area of the insulating substrate is increased.
In an embodiment of the invention, a wiring board has metal interconnections obtained by the above method for fabricating metal interconnections.
According to the embodiment, since the metal interconnections can be made thinner in film thickness, their availability can be improved.
The present invention also provides a method for fabricating metal interconnections, comprising: a first step of forming a ground metal film on an insulating substrate by dry deposition technique; a second step of patterning the ground metal film into a specified pattern; a third step of forming an anti-corrosion metal film on the patterned ground metal film by electroplating technique or electroless selective plating technique; and a fourth step of forming a low-resistance metal film on the anti-corrosion metal film by electroplating technique or electroless selective plating technique.
According to the invention, the dry deposition process has only to be done one time in the first step, and the patterning process (etching process) has only to be done one time in the second step. Therefore, the number of processes involved in the method is smaller than in the metal interconnection fabricating method shown in the prior art example (1) Thus, a cost reduction can be achieved.
Also, the ground metal film is formed on the insulating substrate by using the dry deposition technique instead of the plating deposition technique involving catalyst impartment. Therefore, compared with the metal interconnection fabrication method shown in the prior art example (2), the pre-plating process for performing catalyst impartment is no longer necessary, so that occurrence of minute protrusions on the surface of the ground metal film due to the catalyst can be avoided.
Further, when the electroless selective plating technique is used in the third step and the fourth step, the film thickness uniformity of the anti-corrosion metal film and the low-resistance metal film can be improved over the case where the electroplating technique is used.
In an embodiment of the invention, the ground metal film is a metal film whose principal component is Ni.
According to the embodiment, since the ground metal film is a metal film whose principal component is Ni, the ground metal film can be formed on the insulating substrate simply with good adhesion by using dry deposition technique such as sputtering process or vapor deposition process.
In an embodiment of the invention, the anti-corrosion metal film is a metal film whose principal component is Au.
According to the embodiment, since the anti-corrosion metal film is a metal film whose principal component is Au superior in corrosion resistance, no oxide is formed on the surface of the anti-corrosion metal film. Therefore, no oxide is present between the anti-corrosion metal film and the low-resistance metal film that is formed on the anti-corrosion metal film, so that the low-resistance metal film can be formed on the anti-corrosion metal film with good adhesion by electroplating technique or electroless selective plating technique.
Also, since the anti-corrosion metal film is made principally of low-resistivity Au, the layered film composed of the ground metal film and the anti-corrosion metal film is low in resistance value, so that electric current can be passed more easily through the layered film. Accordingly, with the use of the electroplating technique in the fourth step, since electric current is passed through the layered film composed of the ground metal film and the anti-corrosion metal film, it is advantageous that the layered film is of low resistance. Further, since Au is an expensive noble metal, thinning the anti-corrosion metal film as much as possible and using an inexpensive material for the low-resistance metal film makes it possible to manufacture the metal interconnections with low price.
In an embodiment of the invention, the low-resistance metal film is a metal film whose principal component is Cu.
According to the embodiment, the low-resistance metal film is a metal film whose principal component is Cu, which is low in resistivity (bulk resistivity: 1.7 .mu..OMEGA..multidot.cm) and long in life against electromigration, thus optimal as a material of low-resistance metal interconnections. Also, since Cu is inexpensive, the manufacturing cost for metal interconnections can be reduced.
In an embodiment of the invention, the electroless selective plating technique in the third step is displacement plating.
According to the embodiment, since the electroless selective plating technique in the third step is displacement plating, the pre-processing for performing catalyst impartment is no longer necessary so that the working efficiency in the formation of the anti-corrosion metal film can be improved.
Also, by virtue of the fact that the electroless selective plating technique in the third step is displacement plating, eve n if a natural oxide film is formed at the surface layer of the ground metal film, and if the natural oxide film is thin, the displacement plating solution penetrates through natural oxide film. Therefore, it becomes possible to form an anti-corrosion metal film on the surface of the ground metal film.
In an embodiment of the invention, the method for fabricating metal interconnections further comprises a fifth step of forming a cap film on the low-resistance metal film by electroplating technique or electroless selective plating technique.
According to the embodiment, since the cap film is formed on the low-resistance metal film by electroplating technique or electroless selective plating technique, the low-resistance metal film is not exposed to the atmospheric air so that the low-resistance metal film can be prevented from oxidizing.